There's a little checkbox in your Settings now
that says, "Email me warnings of impending apocalypses." I thought this was
pretty self-explanatory, but there have been questions, so I guess not.
What it does is send you an email seven days before
your nation will cease to exist due to inactivity. Which happens if you
don't log in for 28 days, or 60 days with Vacation mode turned on.

(Deleted nations can be restored again. They just don't exist within the
gameworld in the meantime.)

So basically: the checkbox means you'll get emailed if you don't log in
for 21 days. Or 53 days in Vacation mode.